1. Field
The present disclosure is directed to a method and apparatus for transmitting positioning reference signals in a wireless communication network. More particularly, the present disclosure is directed to preserving orthogonality of positioning reference signals in a network using a mixture of cyclic prefix types.
2. Introduction
Presently, positioning subframes are used by user equipment, such as a mobile terminal, a cell phone, a personal digital assistant, or other user equipment, to determine observed time difference of arrival of positioning subframes from neighbor cells relative to positioning subframes of serving cell using cross-correlation. The observed time difference of arrival is sent to a location server, which computes the location of the user equipment.
Every cell in a network sends out positioning reference signal periodically in positioning subframes. The positioning subframes from the cells are time aligned with other cells for determining the observed time difference of arrival.
A cyclic prefix (CP) can be added to a symbol to reduce or eliminate inter-symbol interference. A Long Term Evolution (LTE) wireless network can use a normal cyclic prefix or an extended cyclic prefix, where normal and extended refer to the length of the cyclic prefix. For example, a normal size cell may use a normal cyclic prefix length because the delay spread of the propagation channel is relatively small. A larger cell may use an extended cyclic prefix length to overcome a larger delay spread in the propagation channel in order to preserve the orthogonality of symbols modulating neighboring subcarriers. A subframe having an extended cyclic prefix can have fewer symbols than a normal cyclic prefix subframe because a subframe can have a fixed length and the extended cyclic prefix symbols are longer than normal cyclic prefix symbols.
In order to perform the cross-correlation, user equipment must be able to construct the reference signal, and to construct the reference signal, the user equipment must know whether the signal is normal cyclic prefix or extended cyclic prefix, because the reference signal time-frequency patterns are different. Assistance data can inform user equipment of whether neighbor cells have the same cyclic prefix or a different cyclic prefix as a reference cell. It is desirable to keep the amount of assistance data low to reduce the amount of data transmitted. Thus, if a neighbor cell has the same cyclic prefix as the reference cell, the bit field indicating the cyclic prefix length is absent. The user equipment infers that the cyclic prefix lengths are identical by suitably parsing the assistance data and determining that the bit field is absent. On the other hand, if the neighbor cell has a different cyclic prefix than the reference cell, a bit can be transmitted with each neighbor cell physical identifier to indicate the cyclic prefix of the neighbor cell relative to the reference cell.
There is a problem when there is a mixed deployment of cells using extended cyclic prefix and cells using normal cyclic prefix because the symbol boundaries do not align between the normal cyclic prefix cells and the extended cyclic prefix. The symbol boundaries do not align because the subframes are fixed in length and the extended cyclic prefix symbols are longer than the normal cyclic prefix symbols. Therefore, extended cyclic prefix symbols have different boundaries than the normal cyclic prefix symbols. This causes a problem when using cross-correlation to determine the observed time difference of arrival of positioning subframes from neighbor cells relative to positioning subframes of serving cell. This problem is exacerbated by the fact that subframes cannot use a mix of extended cyclic prefix and normal cyclic prefix, in part due to such a scheme requiring excessive amounts of assistance data to indicate the cyclic prefix for each subframe.
Thus, there is a need for a method and apparatus that transmits and/or receives positioning reference signals in a wireless communication network using a mixture of cyclic prefix types.